Cytogenetic heteromorphisms and restriction fragment length polymorphisms were used to assign the parental origins of 30 de novo non-homologous Robertsonian translocations. The balanced and unbalanced translocations studied included 20 rob(14q21q) four rob(13q14q)four rob(15q21q) one rob(13q15q), and one rob(13q21q). Significantly more maternally (26/30) than paternally (4/30) derived de novo translocations were noted and all rob(14q21q) ascertained through unbalanced probands (20/20) were maternal in origin. Interestingly, 12/13 probands who were trisomic and informative for proximal chromosome 21q loci were homozygous for the markers tested. Segregation (2:1) of the Robertsonian translocation into one daughter cell in meiosis I and subsequent failure of the chromosome 21 chromatids to separate in meiosis II may account for our observation of homozygosity for proximal chromosome 21 loci in the majority of de novo rearrangements tested.

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